characterization of pure and pb2+ ion doped
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Int. J. Electrochem. Sci., 13 (2018) 11931 – 11952, doi: 10.20964/2018.12.34
International Journal of
ELECTROCHEMICAL SCIENCE
www.electrochemsci.org
Characterization of Pure and Pb2+ ion Doped Methylcellulose
Based Biopolymer Electrolyte Films: Optical and Electrical
Properties
Omed Gh. Abdullah1,2,*, Shujahadeen B. Aziz1,2, Dlear R. Saber1
1 Advanced Materials Research Lab., Department of Physics, College of Science, University of
Sulaimani, 46001, Kurdistan Region, Iraq. 2 Komar Research Center, Komar University of Science and Technology, 46001, Sulaimani, Kurdistan
Region, Iraq. *E-mail: omedghareb@yahoo.com, omed.abdullah@univsul.edu.iq
Received: 26 July 2018 / Accepted: 22 August 2018 / Published: 5 November 2018
Pb2+ ion-conducting biopolymer electrolyte films, based on methylcellulose (MC) were prepared,
using the solution cast technique. The effect of Pb2+ doping concentration on the optical and electrical
properties of host MC are described in this manuscript. Fourier transform infrared (FTIR) spectra
indicated the occurrence of complexation between Pb2+ and biopolymer host in the synthesis
biopolymer-based electrolyte films. Ultraviolet-visible (UV-Vis) spectroscopy accounts for a
considerable continuous decline in optical band gap and band tail energy, which attributed to the
formation of charge-transfer complex and increasing in the crystalline nature of the polymer electrolyte
films, respectively. The dispersion of the refractive index was discussed in term of the single-oscillator
model. The frequency-dependent electrical conductivity and dielectric constants of the prepared
samples were investigated as a function of temperature and frequency by impedance spectroscopy.
Temperature-dependent behavior of the frequency-exponent reveals that the correlated barrier hopping
(CBH) model is the most suitable model to describe the conduction mechanism for the present system.
The highest value of ion conductivity at ambient temperature was found to be 2.68×10−6 S/m for the
polymer incorporated with 20 wt.% Lead acetate. The non-Debye type relaxation behaviour has been
confirmed by the asymmetric relaxation peak of the imaginary part of the electric modulus. The
present biopolymer electrolyte films were specified as promising materials for electrochemical device
applications.
Keywords: biopolymer electrolytes; ion conductivity; charge-transfer complex; band structure;
conduction mechanism
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1. INTRODUCTION
Solid polymer electrolyte materials have attracted considerable attention in view of their
potential applications in various electronic and optoelectronic devices such as solar cells, light-emitting
diodes, gas sensors, supercapacitors, electrochromic displays, fuel cells, and solid state batteries [1-3].
Solid polymer electrolyte based films overcome several limitations and drawbacks of liquid electrolyte
based systems, such as leakage, hazardous gas emission, flammable, and toxicity. High safety,
lightweight, low cost, flexibility, ease thin-film formation of solid polymer electrolytes attract the
interest of both industrial and academic researchers [4,5].
Optical properties of solid polymer electrolytes have been investigated aiming to achieve
remarkable change and tuning the optical properties of host polymer, which are essential for various
applications [6-8]; While intensively investigating the electrical properties are aimed to understand the
origin and nature of charge transport prevalent in these materials [9]. The optical and electrical
properties of solid polymer electrolytes can be suitably tuned by controlling the type and concentration
of the salt-additive [10]. In recent years much research activity has been devoted to the synthesis and
characterization of solid polymer electrolytes made of various synthetic organic and coordination
polymer materials [11-13]. However, these polymers have several disadvantages, such as high cost of
fabrication, and not being environmentally-friendly. As an alternative, several renewable natural
organic polymers such as hydroxyethyl cellulose [14], cellulose [15], chitosan [16], starch [17], and
agar [18] were used as a polymer host for solid polymer electrolytes and showed good ionic
conductivity results. Moreover, natural polymers are a well-known biodegradability, renewability,
environmentally-friendly, low production cost, and versatile physical and chemical properties [2]. For
these reasons, nowadays, the biopolymer electrolyte films have attracted more and more attention
[19,20].
Methylcellulose (MC) is a very interesting, natural organic polymer, water-soluble, not toxic,
renewable, biodegradable, relatively low-cost, and has good film-forming properties [21,22]. To the
best of author’s knowledge, there are no results on the optical and electrical properties of MC doped
Pb2+ ions based biopolymer electrolyte films. Hence, the main objective of this work focuses on the
preparation and characterization of biopolymer electrolyte films based on MC as the host polymer for
lead ions. It is very important to reveal the optical and electrical parameters of MC incorporation with
different filling levels of lead ions, as new material, in order to identify the suitability of the obtained
biopolymer electrolyte films for applications towards technology in optoelectronic and electrochemical
devices.
2. EXPERIMENTAL SECTION
2.1 Preparation of biopolymer electrolyte
Pure and Pb2+ ion doped methylcellulose (MC) based biopolymer electrolyte films were
prepared using solution cast technique. MC was obtained from Sigma-Aldrich. A fixed amount of MC
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(0.5 g) was first dissolved in 25 ml of distilled water under stirring for 48 hours for complete
dissolution of polymer (solution A). On the other hand, different amounts of lead acetate
(Pb(CH3COO)2, 99%, Sigma-Aldrich) according to the 5, 10, 15, and 20 wt.% were dissolved
separately, each in 5 ml of distilled water (solution B). Biopolymer electrolytes were achieved by
mixing solution A and B. The mixture of these solutions were kept on a magnetic stirrer for 30 minutes
at room temperature. Now the obtained homogeneous solutions were cast in plastic Petri dishes, and
the solvent was allowed to evaporate slowly in a closed container at room temperature for films to
form. The dried films were easily peeled off from Petri dishes and coded as BPE-0, BPE-5, BPE-10,
BPE-15 and BPE-20 for MC doped with 0, 5, 10, 15, 20 wt.% of Pb2+ ions, respectively. The thickness
of the obtained free standing-films was in the range of 31 to 38 µm.
2.2 Characterization techniques
The chemical composition of the prepared pure and ion doped MC biopolymer electrolyte films
were tested by Fourier transform infrared (FTIR) spectral analysis using (PerkinElmer FT-IR/NIR
Frontier spectrometer) infrared spectrometer with a resolution of 1.00 cm-1 at room temperature, within
the wave number range of 4000-400 cm-1, to detect any possible interaction between cations and the
polar functional groups on the backbone of host polymer in the prepared polymer electrolyte films.
The optical properties were performed by means of a computerized Ultraviolet-visible (UV-Vis)
double beam spectrophotometer (model: Lambda 25) in the wavelength range 190-1100 nm, with 1 nm
step, at normal incidence at room temperature. Electrical measurements were carried out to determine
the frequency- and temperature-dependence of the electrical conductivity and dielectric response of the
samples. The prepared biopolymer electrolyte films were pressed between two aluminum electrodes (2
cm in diameters) to ensure good contact between the electrodes and the electrolyte. The system was
installed in a Teflon cell, and the measurements were taken using the precision LCR meter (model:
Agilent/HP 4284A), by applying a voltage of 5 mV rms amplitude and varying the frequency from 100
Hz to 1 MHz, and within the temperature range from 30 to 100 oC.
3. RESULTS AND DISCUSSION
3.1 FTIR spectral analysis
In order to identify any possible coordination interaction between the lead acetate salt and the
methylcellulose (MC) polymer chains, FTIR measurements were performed. Figure 1 shows the
comparison of FTIR spectra of pure and Pb2+ ion doped MC as biopolymer electrolyte with 10 and 20
wt.% lead acetate content. It is well known that the observed intense bands at 3446 and 1116 cm−1
assigned, respectively, to the stretching vibrations of hydroxyl groups (O-H), and ether groups (C-O-
C) of pure MC [23]. The typical characteristic bands of MC appeared at 2908 cm−1 indicates the
presence of C-H stretching of CH2 and CH3 groups. The observed strong bands at 1648, and 1412 cm−1
can be ascribed to the C=O stretching, and C-H bending in CH2, respectively [24]. In agreement with
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Rangelova et al. [25] and Filho et al. [26], we also observed characterization bands of pure MC at
1456, 1374, 1316 and 948 cm-1.
Figure 1. FTIR spectra of BPE-0, BPE-10 and BPE-20 films.
The main difference between the absorption spectra of pure and Pb2+ ion doped MC films is the
decrease in the intensity and change in the profile of the observed characteristic band of pure MC film.
The significant shift of O-H peak from 3446 cm−1 for pure MC to 3458 cm-1 for 20 wt.% Pb2+ ion
doped MC, and C=O stretching from 1648 cm−1 for pure MC to 1634 cm-1 for Pb2+ ion doped MC,
could be due to the assignment of hydrogen-bonded and the formation of a charge-transfer complex
between cations and these polar groups of MC [27-29]. The appearance of the 1552 cm-1 peak for
biopolymer electrolyte films, and increasing its intensity with increasing lead acetate concentration is
mainly due to the presence of acetate ions (CH3COO-).
3.2 Ultraviolet-visible spectra analysis
The effects of lead acetate concentration on the optical properties of MC were identified from
the optical absorption spectrum, which is the most direct and simplest method of studying the band
structure of solids [30]. The UV-Vis absorption spectra at normal incidence of light for the pure and
Pb2+ doped MC polymer electrolyte films are shown as a function of wavelengths in Fig. 2.
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Figure 2. Absorption spectra for pure and Pb2+ ion doped MC polymer electrolyte films.
The absorption spectrum of pure MC film is characterized by a sharp absorption edge at 244
nm, which indicates the semi-crystalline nature of MC [31]. The intensity of sharp absorption edge
increase and its position shifted towards longer wavelength, by increasing the Pb2+ concentration in
polymer electrolyte films. This redshift occurs due to the development of strains and the formation of
defects, which result in a decrease in the band gap energy of the prepared films [32].
The dependence of optical absorption coefficient on the incident photon energy helps to
determine the optical band gap energy and the predominant electron transition involved in the
absorption process, by using Tauc’s relation [33]. The optical absorption coefficient (𝛼) is calculated
using the relation, 𝛼 = 2.303𝐴/𝑑, where 𝐴 is the absorbance, and 𝑑 is the film thickness. The theory
of inter-band optical-absorption shows that: the relation between optical absorption coefficient (𝛼) and
incident photon energy (ℎ𝜈) can be expressed as [34,35]:
𝛼ℎ𝑣 = 𝛽(ℎ𝑣 − 𝐸𝑔)𝛾
(1)
where 𝛽 is an appropriate constant, ℎ is Plank’s constant, 𝑣 is the frequency of radiation, and
𝐸𝑔 is the optical energy band gap. Here 𝛾 represents an index that can take any of the values 1/2, 3/2, 2
or 3 depending on the type of electronic transition responsible for the optical absorption. To determine
the most probable electron transition in the present solid biopolymer electrolyte films, the value of 𝛾
has been calculated by taking the natural logarithm and the derivation of Tauc's equation as [36]: 𝑑 𝑙𝑛(𝛼ℎ𝑣)
𝑑 ℎ𝑣=
𝛾
ℎ𝑣 − 𝐸𝑔 (2)
According to this equation, a peak at the value of optical band gap should be observed in the
curve of (𝑑 𝑙𝑛(𝛼ℎ𝑣) /𝑑 ℎ𝑣) versus incident photon energy (ℎ𝑣), as shown in Fig. 3.
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Figure 3. The plot of (𝑑 𝑙𝑛(𝛼ℎ𝑣) /𝑑 ℎ𝑣) versus incident photon energy (ℎ𝑣).
Figure 4. Plot of ln(𝛼ℎ𝑣) versus ln(ℎ𝑣 − 𝐸𝑔) for pure and Pb2+ ion doped MC polymer electrolyte
films.
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Figure 5. Dependence of (αhv)2 on photon energy (ℎ𝑣) for pure and Pb2+ ion doped MC polymer
electrolyte films.
The appearance of a peak at a particular energy value gives the approximate value of optical
band gap. This approximate value of optical band gaps was used to estimate the value of 𝛾, and hence
the nature of electronic transitions [37]. Figure 4 depicted the graph of 𝑙𝑛(𝛼ℎ𝑣) versus 𝑙𝑛(ℎ𝑣 − 𝐸𝑔)
for pure and Pb2+ doped MC biopolymer electrolytes. The value of γ was determined accurately to be
in the range (0.45-0.72). Interestingly, this technique provides evidence for the more probable
electronic transition in the optical bands. This result indicates clearly, the presence of a direct gap i.e.
γ = 0.5.
The appropriate values of direct optical band gap energy can then be calculated using Tauc's
plot. The variation of (𝛼ℎ𝑣)2 with photon energy (ℎ𝑣) for pure MC and MC based biopolymer
electrolyte films are presented in Fig. 5. The intercepts of the best fit line on the energy axis give the
value of direct allowed optical band gap, which tabulated in Table 1.
Table 1. Optical energy results for MC and its biopolymer electrolyte films.
Samples 𝐸𝑔 (𝑒𝑉) 𝐸𝑢 (𝑒𝑉)
BPE-0 6.32 0.256
BPE-5 4.64 0.196
BPE-10 4.54 0.176
BPE-15 4.49 0.173
BPE-20 4.44 0.178
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The value of the optical energy gap for the pure MC film was found to be 6.32 eV, which is
larger than those published previously by other researchers [38]. For the biopolymer electrolyte films,
it was found that 𝐸𝑔 is inversely proportional with lead acetate content. The significant decrease in the
optical energy band gap is an indication of the increase in the electrical conductivity [39], which may
be due to the formation of a charge-transfer complex between lead acetate cation and the hydroxyl
groups of the MC polymer chains [40]. This result is very well supported by FTIR analysis (see Fig.
1).
Enhancing the absorption over the Ultraviolet range and low values of energy band gap of the
present biopolymer electrolyte films confirm the suitability for applications in optoelectronic device
[41].
The absorption coefficient (𝛼) shows an exponential dependence on photon energy (ℎ𝑣) near
the band edge and this dependence is given as Urbach empirical rule [42]:
𝛼 = 𝛼𝑜 exp (ℎ𝑣
𝐸𝑢) (3)
where 𝛼𝑜 is a constant and, 𝐸𝑢 is width of the tail of localized state in the band-gap and is
known as the Urbach energy. The values of 𝐸𝑢 give more detail about the optical behavior of the pure
and polymer electrolyte films [39]. Figure 6(a,b) shows the variation of ln(𝛼) with respect to the
photon energy (ℎ𝑣) at different content of lead acetate in biopolymer electrolyte films. The values of
the Urbach energy were determined as the reciprocal of the straight lines slopes shown in Fig. 6, the
obtained values have been presented in Table 1.
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Figure 6. Plot of 𝑙𝑛(𝛼) versus incident photon energy ℎ𝑣 for: (a) pure MC, and (b) lead ion doped MC
biopolymer electrolyte films.
Table 2. The dispersion parameters 𝐸𝑑, 𝐸𝑜 for pure and Pb2+ doped MC biopolymer electrolyte films.
Samples 𝐸𝑑 (𝑒𝑉) 𝐸𝑜 (𝑒𝑉) 𝐸𝑜/𝐸𝑔
BPE-0 1.814 6.265 0.991
BPE-5 0.933 4.703 1.013
BPE-10 1.068 4.683 1.031
BPE-15 1.253 4.668 1.039
BPE-20 1.561 4.676 1.053
It can be seen that the Urbach energy of biopolymer electrolyte films gradually decreases with
increase in Pb2+ ion content. Thus, the decrease in 𝐸𝑢 from 0.25 to 0.17 eV as the lead ion content
increases from 0 to 20 wt.% describes the decrease of band tails.
The decrease of band tails may be due to increase in the crystalline nature of the polymer
electrolyte. It is well known that the shape of the fundamental optical absorption edge in the
exponential region can yield information on the disorder effects [43]. The increase and sharpness of
the absorption coefficient in the exponential region for biopolymer electrolyte as shown in Fig. 2,
ascribes to the decay of density of the localized states in the optical band gap [44].
The refractive index (𝑛) is the most important optical parameter for optoelectronics application
due to its direct relation to the physical, chemical, and molecular properties of materials [45]. The
value of 𝑛 can be determined from reflectance (𝑅) and extinction coefficient (𝑘 = 𝜆𝛼/4𝜋) based on
the known relationship [46]:
𝑛 =1 + 𝑅
1 − 𝑅+ √
4𝑅
(1 − 𝑅)2− 𝑘2 (4)
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The dispersion of the refractive index of the present biopolymer electrolyte films has been
analyzed using the single-oscillator model, which is expressed by the Wemple-DiDomenico relation
[47] 1
(𝑛2 − 1)=
𝐸𝑜
𝐸𝑑−
1
𝐸𝑜𝐸𝑑
(ℎ𝑣)2 (5)
where 𝐸𝑜 is the single-oscillator energy, 𝐸𝑑 is the dispersion energy. In practice, the dispersion
parameters 𝐸𝑜 and 𝐸𝑑 can be determined from a linear fit parameters of the plot of 1/(𝑛2 − 1) against
(ℎ𝜈)2 as shown in Fig. 7.
Figure 7. The relation between 1/(𝑛2 − 1) and (ℎ𝜈)2 for prepared films.
The values obtained for the dispersed parameters 𝐸𝑜 and 𝐸𝑑 are listed in Table 2. The value of
oscillator energy 𝐸𝑜 is related to the optical energy band gap 𝐸𝑔 [48]. Empirically the 𝐸𝑜 values of the
present films is almost equal to the optical band gap 𝐸𝑜 ≈ 𝐸𝑔 as shown in Table 2. The 𝐸𝑑 value is a
measure of the strength of inter-band optical transitions, and it was well reported in the literatures that
the ordering of the film structure leads to an increase in 𝐸𝑑 [49]. This result is also compatible with the
results of tail of localized state tabulated in the Table 1.
3.3 Electrical conductivity analysis
Several models are present in the literature to describe the source and nature of charge carriers
in polymeric systems, which take into account the particular electronic structure of a given polymer
[50]. The measurement of the ac conductivity as a function of frequency, temperature, and composition
provides very important information about the nature of the conduction process in materials [51].
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For clarity, the plot of the frequency dependence of ac conductivity (𝜎𝑎𝑐) at different
temperatures is displayed in Fig. 8(a,b) for pure and Pb2+ ion doped MC samples with 20 wt.%
Pb(CH3COO)2.
Figure 8. The frequency dependence of the ac conductivity at different temperatures for (a) pure MC,
(b) polymer electrolyte doped with 20 wt.% Pb(CH3COO)2.
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Figure 9. Log-log plot of ac conductivity versus angular frequency for (a) pure MC, (b) polymer
electrolyte doped with 20 wt.% Pb(CH3COO)2 at a different temperature.
The frequency response of the curves indicate the existence of low frequency independent
plateau-like region due to dc conductivity (𝜎𝑑𝑐) and subsequently, the conductivity increases with an
increase in frequency, which represents a bulk relaxation phenomenon [52]. Also, it can be seen, each
curve shows conductivity dispersion, which is strongly affected by frequency, temperature and Pb2+
ion concentration. The value of 𝜎𝑎𝑐 increases monotonically with increasing frequency and
temperature. This type of behavior reveals that the dominant current transport mechanism could be due
to hopping [53]. Thus, the increase of the applied frequency improves the electronic jumps between the
localized states; consequently, the ac conductivity increases with increasing frequency [54]. On the
other hand, the temperature dependence of the ac conductivity can be explained as follows: the
polymer chain mobility increases as the temperature increases, and the fraction of free volume inside
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the polymer matrix increases accordingly. This facilitates the segmental motion of the polymer chains
and leads to enhancement in 𝜎𝑎𝑐 at high temperatures [55]. Moreover, the increase in the ionic
conductivity by adding lead salt is attributed to the increase in the number of Pb2+ ions as well as their
mobility in the polymer host.
It is clear from the curves in Fig. 8 that the ac conductivity has a frequency dependence given
by the empirical Jonscher's universal power law [46]
𝜎𝑎𝑐 = 𝐵𝜔𝑠 (6)
where 𝐵 is a per-exponential factor depending on temperature, 𝜔 = 2𝜋𝑓 is the angular
frequency and, 𝑠 is the dimensionless frequency-exponent, generally less than or equal to unity
(0 < 𝑠 < 1). At lower frequencies, ions travel much slower and are able to jump from one available
site to another vacant site in the host polymer matrix, and contributes to the dc conductivity. At higher
frequencies, the ions perform correlated forward-backward hopping motions on small length scales.
The movements are successful when ions jump and stay in the new site. Whereas the movements are
unsuccessful hopping if the jumped ion jump back to its initial position [56]. As the ratio of successful
to unsuccessful hopping increases, results in more dispersive conductivity at higher frequencies [57].
Table 3. The variation of frequency-exponent (𝑠) with temperature for all samples.
T (K)
(s)
BPE-0 BPE-5 BPE-10 BPE-15 BPE-20
295 0.776 0.677 0.445 0.321 0.565
303 0.733 0.672 0.349 0.304 0.554
313 0.695 0.582 0.311 0.194 0.425
323 0.601 0.502 0.261 0.134 0.277
333 0.500 0.426 0.227 0.090 0.209
343 0.446 0.320 0.197 0.066 0.147
353 0.428 0.286 0.172 0.060 0.116
363 0.413 0.254 0.153 0.061 0.098
373 0.407 0.247 0.137 0.053 0.098
To calculate the frequency-exponent (𝑠), the double logarithmic plots of the ac conductivity
versus angular frequency at different temperatures for pure MC and MC doped with 20 wt.% lead salt
are presented in Fig. 9. It is clear from Fig. 9 that the variation in the log(𝜎𝑎𝑐) is almost linear with the
variation in log(𝜔) in the frequency range 4.80 ≤ log(𝜔) ≤ 6.80. The frequency-exponent (𝑠) have
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been estimated from the slope of the least-square straight-line fits of the experimental data for all
temperatures, and is listed as a function of temperature in Table 3. It is evident that the values of 𝑠
decrease continuously with increasing temperature for both pure and Pb2+ ion doped MC films.
Various theoretical models such as the hoping over a barrier (HOB), correlated barrier hopping
(CBH), quantum mechanical tunnelling (QMT), small polaron tunneling model (SPT), overlapping
large polaron tunneling model (OLPT), etc., have been proposed to explain the conduction mechanism
based on the variation of frequency-exponent (𝑠) with temperature. The temperature dependence of the
𝑠 suggests that the CBH model may be suitable to explain the conduction mechanism in the studied
temperature region [9]. In essence, the CBH model considers hopping of carriers between two sites
over a potential barriers separating them, rather than tunneling the barrier [58]. According to this
model, the values of 𝑠 has a maximum value at room temperature, and have a tendency to decrease
with increasing temperature [53].
The value of activation energies (𝐸𝑎) have been estimated from the relationship between dc
conductivity and temperature, using the Arrhenius relation [52]:
𝜎𝑑𝑐 = 𝜎𝑜 exp(−𝐸𝑎/𝑘𝐵𝑇) (7)
where 𝜎𝑜 is a pre-exponential factor, 𝑘𝐵 is Boltzmann's constant, and 𝑇 is temperature in
Kelvin. The dc conductivity values were estimated from the extrapolation of the plateau region of ac
conductivity spectra shown in Fig. 8.
Figure 10. Variation of log 𝜎𝑑𝑐 as a function of 1000/𝑇 for pure and doped MC with Pb(CH3COO)2.
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The conductivity of 20 wt.% Pb+2 ion doped film exhibits the highest ion conductivity
(2.68×10−6 S/m) compared with other electrolyte films, which is due to the increased Pb2+ ion within
the electrolyte system. The maximum ion conductivity at ambient temperature obtained in this study is
higher than that obtained by Krishnakumar et al. [59] (2.264×10−7 S/m) for Pb+2 ion doped PVA-PEG
polymer composite electrolyte films. The variation of dc conductivity versus reciprocal of temperature
for present solid polymer electrolyte films, treated as a single straight line region with a single
activation energy value, as shown in Fig. 10. The activation energy (𝐸𝑎) values is extracted from the
slope of the plot log(𝜎𝑑𝑐) verses 1000/𝑇. The variation of 𝐸𝑎 as a function of lead ion concentration
are depicted in Fig. 11. It is noticed that the value of 𝐸𝑎 is inversely proportional to the lead ion
content. Therefore, the highest ion content, have the lowest activation energy, and hence maximum dc
conductivity.
Figure 11. Variation of activation energy and dc-conductivity versus lead salt concentration.
3.4 Dielectric constant analysis
The dielectric polarization in polymer electrolyte systems may be due to several microscopic
processes, such as dipole alignment, rotation/motion of the segmental polymer chain, migration of
ions, the drift of electrons or holes, and charge injection from the electrodes [31,50]. Methylcellulose
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(MC) belong to the group of polyglycans, carrying two hydroxyl side groups (-OH) and one
hydroxymethyl side group (-CH2-OH). These permanent dipolar moments of the attached side groups
will be the source of the dielectric relaxation.
Figure 12 shows the frequency-dependence of the dielectric constant (𝜖′) at different
temperatures for pure and doped MC samples with 20 wt.% Pb(CH3COO)2. A high value of 𝜖′ has
been observed in the case of Pb2+ ion doped polymer electrolyte films in comparison with the pure MC
film. The overall increase in dielectric constant with increasing Pb2+concentration, indicates
improvement in the polarizability contributions. Moreover, the dielectric constant (𝜖′) decreases with
increasing applied frequency at given temperature because the permanent dipoles are no longer able to
keep up with changes in the applied electric field, consequently the number of orientational
polarization of polar molecules decreases at higher frequencies [9]. It is worth noting that the value of
the dielectric constant (𝜖′) is also increases with increasing the temperature. This is because the
orientation polarization is related to the thermal motion of the molecules [60]. At lower temperatures,
the absorbed thermal energy by the polymeric material, at a certain frequency is small, so that a small
number of dipoles per unit volume can orient themselves in the direction of the applied electric field
[55]. As the temperature increases, the viscosity of the polymeric material is decreased, and the dipoles
have enough energy to orient themselves easily in the direction of the applied electric field, and thus
the orientation polarization increases, which consequently increases the dielectric constant [60,61].
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Figure 12. Variation of dielectric constant with frequency at different temperature for (a) BPE-0, and
(b) BPE-20.
Based on the obtained results, the optical and dielectric properties of MC can be modified
significantly by introducing Pb2+ ions. Therefore, MC/Pb(CH3COO)2 biopolymer electrolyte is a
candidate for several industrial and electronic devices with improved the dielectric strength and charge
storage capacity.
3.5 Electrical Modulus analysis
The electric modulus (reciprocal quantity of dielectric permittivity), has been extensively used
to analyze and interpret the dielectric response caused by charge relaxation, in which the electrode
polarization effects are suppressed [62]. The modulus formalism provides wider insights into ion
dynamics processes as a function of frequency at a different temperature. Figure 13 showed the
frequency-dependence of the real and imaginary parts of the complex electrical modulus, (M′ and M′′),
for 20 wt.% Pb2+ ion doped MC (BPE-20), at different temperature.
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Figure 13. Variation of (a) real and (b) imaginary parts of complex electrical modulus with frequency
at different temperature for BPE-20.
The plot of M′ displays low value at lower frequencies due to the large value of capacitance
associated with the electrodes. The observed dispersion of M′ with frequency is ascribed to
conductivity relaxation which accompanied by a loss peak in the diagram of M′′ versus frequency. The
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reduction in M′ with increasing temperature is related to the segmental chain motion due to an increase
in free volume caused by the thermal expansion [24]. The M′′ spectra in Fig. 13-b shows a single
relaxation peak that converges at the dispersion region of M′. The asymmetric and broadening in M′′
peak is indicative of a non-Debye type relaxation behavior in the present electrolyte system [63]. The
observed low frequencies relaxation peak is shifted toward the higher frequencies side with an increase
in temperature. This revealed that relaxation time for charge carriers motion decreases with increasing
temperature due to the increase in carrier’s mobility and dipolar orientation [24].
In polymer electrolyte systems more than one type of polarization mechanism occurs due to
interactions between ions and polar groups of the polymer backbone. Thus there were distributions of
relaxation time. The study of Argand diagrams (M′ vs. M′′) at different temperatures can be used to
provide insight into the nature of the relaxation processes in the present biopolymer electrolyte system.
It was well reported that, if Argand plot shows semicircular behavior, then the relaxation is due to
conductivity relaxation process. Otherwise, it is due to viscoelastic relaxation [64].
Figure 14 shows the Argand plots at different temperature for 20 wt.% Pb2+ ion doped MC
polymer electrolyte film. It can be seen clearly, that the Argand plots of this sample exhibit incomplete
semicircular arcs, indicating, non-Debye relaxation, and ion transport occurs due to carrier relaxation
process with a single value of relaxation time, at the frequency-domain range [16]. Moreover, it can be
seen that with increasing temperature, the relaxation peak shifts toward the origin, confirming the
increase in conductivity due to the thermal activation process.
Figure 14. Argand plots spectra at different temperature for BPE-20.
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4. CONCLUSION
The eco-friendly biopolymer electrolyte films were prepared by incorporating various
compositions of lead acetate with MC biopolymer using the solution cast technique. The dependence
of the fundamental optical and electrical parameters of the MC polymer electrolyte on Pb2+ ion content
was investigated. FTIR studies confirm the complexation due to the interaction of the lead cation with
the functional groups on the backbone of host polymer. The increase of optical absorption in the
ultraviolet region of this system is due to the excitations of donor electrons to the conduction band at
these energies. The direct transition band gap for biopolymer electrolyte films decreases with
increasing the Pb2+ ion content. This behavior may be associated with the structural changes occurring
after the addition of lead acetate salt. The decrease of Urbach energy of polymer electrolyte is obtained
upon addition of lead ions, which correlated with the decrease of disorder in the system. The
temperature dependence of the frequency-exponent (𝑠) of the present biopolymer electrolyte systems,
reveal that correlated barrier hopping (CBH) model is the dominant conduction mechanism in the
MC/Pb(CH3COO)2 system. The maximum ionic conductivity of 2.68×10−6 S/m at ambient temperature
was observed for the sample containing 20 wt.% Pb(CH3COO)2. The electrical modulus analysis
demonstrated a non-Debye type of relaxation process.
ACKNOWLEDGEMENT
The authors gratefully acknowledge the financial support from the Kurdistan National Research
Council (KNRC)- Ministry of Higher Education and Scientific Research-KRG, Iraq. The authors
would also appreciatively acknowledge the support received from the University of Sulaimani, and the
Komar University of Science and Technology for carrying out this work.
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